Quantum-Size Effects in the Visible Photoluminescence of Colloidal ZnO Quantum Dots: A Theoretical Analysis

In this work we develop a simple theory for the green photoluminescence of ZnO quantum dots (QDs) that allows us to understand and rationalize several experimental findings on fundamental grounds. We calculate the spectrum of light emitted in the radiative recombination of a conduction band electron with a deeply trapped hole and find that the experimental behavior of this emission band with particle size can be understood in terms of quantum size effects of the electronic states and their overlap with the deep hole.We focus the comparison of our results on detailed experiments performed for colloidal ZnO nanoparticles in ethanol and find that the experimental evolution of the luminescent signal with particle sizeat room temperature can be better reproduced by assuming the deep hole to be localized at the surface of the nanoparticles. However, the experimental behavior of the intensity and decay time of the signal with temperature can be rationalized in terms of holes predominantly trapped near the center of the nanoparticles at low temperatures being transferred to surface defects at room temperature. Furthermore, the calculated values of the radiative lifetimes are comparable to the experimental values of the decay time of the visible emission signal.We also study the visible emission band as a function of the number of electrons in the conduction band of the nanoparticle, finding a pronounced dependence of the radiative lifetime but a weak dependence of energetic position of the maximum intensity.Comment: 6 figure

Diffuse Surface Scattering in the Plasmonic Resonances of Ultra-Low Electron Density Nanospheres

Localized surface plasmon resonances (LSPRs) have recently been identified in extremely diluted electron systems obtained by doping semiconductor quantum dots. Here we investigate the role that different surface effects, namely electronic spill-out and diffuse surface scattering, play in the optical properties of these ultra-low electron density nanosystems. Diffuse scattering originates from imperfections or roughness at a microscopic scale on the surface. Using an electromagnetic theory that describes this mechanism in conjunction with a dielectric function including the quantum size effect, we find that the LSPRs show an oscillatory behavior both in position and width for large particles and a strong blueshift in energy and an increased width for smaller radii, consistent with recent experimental results for photodoped ZnO nanocrystals. We thus show that the commonly ignored process of diffuse surface scattering is a more important mechanism affecting the plasmonic properties of ultra-low electron density nanoparticles than the spill-out effect.Comment: 19 pages, 5 figures. Accepted for publication in The Journal of Physical Chemistry Letter

Resonant coupling between localized plasmons and anisotropic molecular coatings in ellipsoidal metal nanoparticles

We present an analytic theory for the optical properties of ellipsoidal plasmonic particles covered by anisotropic molecular layers. The theory is applied to the case of a prolate spheroid covered by chromophores oriented parallel and perpendicular to the metal surface. For the case that the molecular layer resonance frequency is close to being degenerate with one of the particle plasmon resonances strong hybridization between the two resonances occur. Approximate analytic expressions for the hybridized resonance frequencies, their extinction cross section peak heights and widths are derived. The strength of the molecular - plasmon interaction is found to be strongly dependent on molecular orientation and suggest that this sensitivity could be the basis for novel nanoparticle based bio/chemo-sensing applications.Comment: 11 pages, 5 figure

A Mechanism for Cutting Carbon Nanotubes with a Scanning Tunneling Microscope

We discuss the local cutting of single-walled carbon nanotubes by a voltage pulse to the tip of a scanning tunneling microscope. The tip voltage ($\mid V \mid \ge$~3.8 eV) is the key physical quantity in the cutting process. After reviewing several possible physical mechanisms we conclude that the cutting process relies on the weakening of the carbon-carbon bonds through a combination of localized particle-hole excitations induced by inelastically tunneling electrons and elastic deformation due to the electric field between tip and sample. The carbon network releases part of the induced mechanical stress by forming topological defects that act as nucleation centers for the formation of dislocations that dynamically propagate towards bond-breaking.Comment: 7 pages, 6 postscript figures, submitted to PR

Wounds as probes of electrical properties of skin

We have built a model where we use a wound as a probe of the dielectric properties of skin. In this way one is able to infer infor- mation about skin dielectric properties in situ. We introduce the notion of a skin electrochemical capacitor. This gives good agreement with recent measurements for the electric potential landscape around a wound. Possible diagnostic consequences are briefly touched upon

Circularly polarized light emission in scanning tunneling microscopy of magnetic systems

Light is produced when a scanning tunneling microscope is used to probe a metal surface. Recent experiments on cobalt utilizing a tungsten tip found that the light is circularly polarized; the sense of circular polarization depends on the direction of the sample magnetization, and the degree of polarization is of order 10 %. This raises the possibility of constructing a magnetic microscope with very good spatial resolution. We present a theory of this effect for iron and cobalt and find a degree of polarization of order 0.1 %. This is in disagreement with the experiments on cobalt as well as previous theoretical work which found order of magnitude agreement with the experimental results. However, a recent experiment on iron showed 0.0 ${\pm}$2 %. We predict that the use of a silver tip would increase the degree of circular polarization for a range of photon energies.Comment: 15 pages, 4 figures, (To appear in Phys. Rev. B, February 2000

"Aina on kiva kuvata" : Työpajanuorten innostamista valokuvatyöskentelyn avulla

Opinnäytetyössä selvitetään työpajanuorten motivaation tukemista ja innostamista valokuvauksen keinoin. Opinnäytetyö toteutettiin osana Matti - miesten matkat työhön -hanketta, jonka tavoitteena on kehittää voimavaroja edistäviä ohjausmalleja työpajatoiminnassa oleville 15-29-vuotiaille miehille. Opinnäytetyössä kehitetään ja sovelletaan valokuvamenetelmää työpajanuorten osaamisen kokemuksen, osallisuuden ja itseohjautuvuuden vahvistamiseksi. Matti-hankkeen kannalta keskeistä on motivoida nuoria hakeutumaan koulutukseen tai työelämään. Opinnäytetyön teoreettinen lähtökohta on itseohjautuvuusteoria. Teoriassa katsotaan yhden kolmesta psyykeen perustarpeesta olevan tunne siitä, että yksilön toiminta on lähtöisin hänen omista haluistaan ja tarpeistaan. Työn toinen teoreettinen lähtökohta on sosiokulttuurinen innostaminen, jossa ihmisen herättämisen ja liikkeelle saamisen avulla pyritään elämänlaadun parantumiseen. Toiminnallinen osuus järjestettiin Helsingissä nuorten työpajalla kahdella toimintakerralla. Ensimmäiseen työpajaan osallistui kuusi ja toiseen kolme nuorta. Tutkimusaineisto koostui nuorten havainnoinnista ja suullisesta palautteesta, sekä hankkeen koordinaattorin haastattelusta. Tuloksista kävi ilmi, että valokuvaus innosti nuoria tarkastelemaan ja ilmaisemaan omia vahvuuksiaan. Myös kirjallisten lämmittelytehtävien havaittiin helpottuvan, kun valokuvaus liitettiin toimintaan. Nuoret innostuivat jatkamaan valokuvaamista myös varsinaisen toimintakerran päätyttyä. Käytetyistä menetelmistä innostavin oli jälkimmäisellä toimintakerralla käytetty pikafilmikamera. Haastavimmiksi osoittautuivat lämmittelynä pidetyt erilaiset kirjalliset ja suulliset harjoitteet. Valokuvaus osoittautui hankkeen tarpeita vastaavaksi menetelmäksi ja yhtenä opinnäytetyön tuloksena on malli siitä, miten valokuvaus voidaan linkittää osaksi työpajojen normaalia toimintaa. Tuloksia voidaan hyödyntää Matti -hankkeen työpajojen toimintamallien suunnittelussa esimerkiksi ottamalla valokuvaus mukaan toimintaan alusta alkaen ja menetelmäksi työpajanuorten oman edistymisen seuraamiseksi.The aim of our practice-based thesis was to investigate supporting the motivation and animation of young men in a workshop using photographic methods. Our thesis was part of Matti - Men’s journeys to work project, which develops operational models to strengthen the resources of young men, aged 15-29. Our thesis develops and applies photographic methods in order to reinforce the feelings of ability, communality and self-determination as a part of operational models used by the Matti project. The main focus of the Matti project, namely the motivation of young people to seek education and work. We chose the self-determination theory and sociocultural animation as our theoretical basis. The self-determination theory considers one of the basic needs of the human psyche to be the need for self-expression. The other basis for our thesis is sociocultural animation, which views awakening and activating people as a method for improving their quality of life. The practical part of our thesis was organized in a workshop for young people in Helsinki. Our first session had six participants and the second one had three. The material used in our thesis is based on observing the participants, oral feedback and interviewing the project coordinator who was present on both occasions. The results indicate that photographic methods animated young people to reflect and express their strengths. The written tasks were also easier to carry out once photography was involved in the task. The young people also continued to take photographs independently after the session was over. Out of the used methods, the most successful was the instant-film camera used in the second session. The most challenging was the written and oral tasks used as a warm-up exercise. The results show that photography is a good method for the Matti project and can also be added as a part of regular workshop activities. The results can be used in further developing operational models used in the Matti project